Power conversion is often desired for providing particular power sources, for portability of power sources, for providing power in bulk, for transmission, or for other reasons. As one example, a major initiative has been taken recently towards the development of a modular solid-oxide-fuel-cell (SOFC) based inverter power system, which mainly feeds bulk power to a local utility load and, unlike active filters, does not need to support more than 30° of load power factor. However, various barriers exist to efficient and effective power conversion.
For example, isolation between a power source and a load is highly desirable and often a necessity (e.g., for safety reasons). To provide such isolation, magnetic transformers may be used. Such transformers, though, can be quite large (e.g., tens of tons), bringing difficulties for installation, repair, etc.
An alternative type of power conversion uses a high-frequency transformer, which can provide isolation while reducing the size of the transformer by a significant amount. High-frequency transformers have become more feasible due to recent technical advances. One example is a high-frequency inverter power system, which reduces the huge cost associated with the labor, weight, and footprint of a traditional utility transformer. The potential for high-frequency transformers has been improved by nanocrystalline core based transformer technology and high-voltage and high-frequency Si IGBTs and (more recently) SiC MOSFETs.
Towards that goal, two example feasible high-frequency, high voltage topologies have received attention in the art. One topology 10 is shown in FIG. 1, and is referred to herein as direct-power-conversion (DPC) topology. Another topology, referred to herein as voltage-source inverter (VSI) topology, may be obtained from the DPC topology 10 in FIG. 1 by simply placing a link capacitor between output converter and the rectifier. Both the DPC and VSI topologies can support varying load power factor angle up to, as a nonlimiting example, 60 degrees or more.
Referring to FIG. 1, both topologies have a front-end high-frequency converter 12 and a diode-rectifier bridge 14, and satisfy a requirement for isolation of a galvanic transformer 16 for a power source 18, such as but not limited to a fuel cell, battery, photovoltaic cell, etc. In other embodiments, the rectifier bridge may be switch-based to allow bidirectional power flow.
However, the two topologies differ at the output stage. An output stage of the VSI topology is a dc/ac inverter that is preceded by a decoupling link capacitor, while the last stage of the DPC topology is an ac/ac converter 20, which is not preceded by any decoupling link capacitor. Further, for the DPC topology, the primary-side converter 12 is operated with sine-wave modulation. For the VSI topology, on the other hand, the primary-side converter operates with square-wave modulation and the output (dc/ac) converter is sinusoidally modulated. In VSI, the filter capacitor provided after the rectifier feeds the output converter with a dc voltage rather than a pulsating dc.
In spite of advances in power conversion, several problems remain. One significant problem is switching loss of the converter. Another problem is the significant stresses (e.g., voltage, current stresses, electromagnetic emissions) on high-frequency, high-voltage devices.